1) Field of the Invention
The invention relates to the supply of a gas to a torch such as a plasma arc torch and, more particularly, to a system and method for controlling the flow of the gas according to the operating parameters of the torch.
2) Description of Related Art
Plasma arc devices are commonly used for cutting and welding. One conventional plasma arc torch includes an electrode positioned within a nozzle. A pressurized gas is supplied to the torch and flows through the nozzle and proximate to the torch, and an arc is established between the electrode and a workpiece. For example, according to one typical method for starting the torch, a pilot mode is first initiated by establishing an arc at a low current between the electrode and the nozzle. A flow of gas is also delivered through the nozzle during the pilot mode. The torch is then switched from the pilot mode to a transfer or working mode by transferring the arc to the workpiece so that the arc extends between the electrode and the workpiece. The current of the arc is increased for the working mode, and the flow rate or type of the gas can also be adjusted. The arc ionizes the gas, and the resulting high temperature gas can be used for cutting or other welding operations. One such torch and a start-up operation are further described in U.S. Pat. No. 5,017,752, titled “Plasma arc torch starting process having separated generated flows of non-oxidizing and oxidizing gas,” issued May 21, 1991, which is assigned to the assignee of the present invention and the entirety of which is incorporated herein by reference.
A metering system supplies the gas that flows through the nozzle of the torch. One conventional metering system operates by attempting to provide a constant pressure in the gas that is delivered to the torch. That is, the metering system is disposed between a source of the gas and the torch and controls the flow of the gas from the source to the torch according to the pressure downstream of the metering system. For example, the metering system can be a mechanical pressure regulator or a proportional valve that receives a feedback signal indicative of the pressure of the gas flowing from the metering system to the torch. Constant pressure metering systems typically provide quick response. That is, the pressure can be changed relatively quickly at start-up or otherwise as desired. However, the flow rate provided by a constant pressure metering system can vary, thereby affecting the performance of the torch and possibly increasing wear on the components of the torch.
Another conventional metering system is configured to provide a constant flow rate of the gas to the torch. For example, a constant flow rate metering system can include a throttle valve for controlling the flow of the gas from the gas source to the torch. If the throttle valve receives the gas at a constant upstream pressure from the gas source, and the downstream pressure is sufficiently less than the upstream pressure, the flow through the throttle valve will be constant. Constant flow metering can also be achieved by providing a constant upstream pressure through a proportional valve or a fixed orifice. A bank of selectable orifices can be provided so that different flow rates can be achieved during different modes of operation of the torch. Alternatively, a fixed orifice can selectively receive different upstream pressures according to the desired flow rate. In any case, constant flow metering generally provides consistent torch performance, and can also provide optimum life of the torch components. However, the time required to stabilize a system with constant flow can be considerable. That is, although the flow rate may be constant through the metering system, the flow rate and pressure downstream in the torch may vary asymptotically during transient modes of operation, such as during start-up of the torch. In particular, after the metering system begins to provide a constant flow of gas, the flow path for the gas between the metering system and the torch may take 10 seconds or longer to achieve a stabilized pressure, during which time the torch may not operate efficiently.
Regardless of the type of metering system, the electrode can become eroded during operation. Erosion can be minimized, for example, by supplying a non-oxidizing gas to the torch during certain modes of operation. However, the electrode can still be eroded, especially if subjected to stress, repeated starting and stopping, or the like. In particular, the electrode can deteriorate quickly if a double arc exists, i.e., if arcs simultaneously exist between the electrode and the nozzle and between the electrode and the workpiece.
Thus, there exists a need for an improved metering system and associated method for providing gas to a torch. The system should be capable of providing the gas at a substantially constant flow rate. Additionally, the system should be capable of changing the flow rate at the torch relatively quickly, such as at start-up of the torch or during other times when it is desirable to adjust the rate of flow according to the operation of the torch. Further, the system should optionally provide detection of particular modes of operation of the torch that may be undesirable, such as the existence of double arcs.